Recovering hydrocarbons from subterranean zones relies on the process of drilling wellbores.
Wellbores are made using surface-located drilling equipment which drives a drill string that eventually extends from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. Drilling fluid usually in the form of a drilling “mud” is typically pumped through the drill string. The drilling fluid cools and lubricates the drill bit and also carries cuttings back to the surface. Drilling fluid may also be used to help control bottom hole pressure to inhibit hydrocarbon influx from the formation into the wellbore and potential blow out at the surface.
Bottom hole assembly (BHA) is the name given to the equipment at the terminal end of a drill string. In addition to a drill bit a BHA may comprise elements such as: apparatus for steering the direction of the drilling (e.g. a steerable downhole mud motor or rotary steerable system); sensors for measuring properties of the surrounding geological formations (e.g. sensors for use in well logging); sensors for measuring downhole conditions as drilling progresses; one or more systems for telemetry of data to the surface; stabilizers; heavy weight drill collars, pulsers and the like. The BHA is typically advanced into the wellbore by a string of metallic tubulars (drill pipe).
Modern drilling systems may include any of a wide range of electronics systems in the BHA or at other downhole locations. Such electronics may include sensors for collecting data of various kinds, controls for downhole equipment, signal processing systems, data telemetry systems etc. Supporting and protecting downhole electronics is important as a downhole electronics package may be subjected to high pressures (20,000 p.s.i. or more in some cases), along with severe shocks and vibrations.
There are references that describe various centralizers that may be useful for supporting a downhole electronics package centrally in a bore within a drill string. The following is a list of some such references: US2007/0235224; US2005/0217898; U.S. Pat. Nos. 6,429,653; 3,323,327; 4,571,215; 4,684,946; 4,938,299; 5,236,048; 5,247,990; 5,474,132; 5,520,246; 6,429,653; 6,446,736; 6,750,783; 7,151,466; 7,243,028; US2009/0023502; WO2006/083764; WO2008/116077; WO2012/045698; and WO2012/082748.
U.S. Pat. No. 5,520,246 issued May 28, 1996 discloses apparatus for protecting instrumentation placed within a drill string. The apparatus includes multiple elastomeric pads spaced about a longitudinal axis and protruding in directions radially to the axis. The pads are secured by fasteners.
US 2005/0217898 published Oct. 6, 2005 describes a drill collar for dampening downhole vibration in the tool-housing region of a drill string. The collar has a hollow cylindrical sleeve having a longitudinal axis and an inner surface facing the longitudinal axis. Multiple elongate ribs are mounted to the inner surface and extend parallel to the longitudinal axis.
Telemetry information can be invaluable for efficient drilling operations. For example, telemetry information may be used by a drill rig crew to make decisions about controlling and steering the drill bit to optimize the drilling speed and trajectory based on numerous factors, including legal boundaries, locations of existing wells, formation properties, hydrocarbon size and location, etc. A crew may make intentional deviations from the planned path as necessary based on information gathered from downhole sensors and transmitted to the surface by telemetry during the drilling process. The ability to obtain and transmit reliable data from downhole locations allows for relatively more economical and more efficient drilling operations.
Various techniques have been used to transmit information from a location in a bore hole to the surface. These include transmitting information by generating vibrations in fluid in the bore hole (e.g. acoustic telemetry or mud pulse telemetry) and transmitting information by way of electromagnetic signals that propagate at least in part through the earth (EM telemetry). Other telemetry systems use hardwired drill pipe, fibre optic cable, or drill collar acoustic telemetry to carry data to the surface.
A typical arrangement for electromagnetic telemetry uses parts of the drill string as an antenna. The drill string may be divided into two conductive sections by including an insulating joint or connector (a “Gap sub”) in the drill string. The gap sub is typically placed at the top of a bottom hole assembly such that metallic drill pipe in the drill string above the BHA serves as one antenna element and metallic sections in the BHA serve as another antenna element. Electromagnetic telemetry signals can then be transmitted by applying electrical signals between the two antenna elements. The signals typically comprise very low frequency AC signals applied in a manner that codes information for transmission to the surface. The electromagnetic signals may be detected at the surface, for example by measuring electrical potential differences between the drill string or a metal casing that extends into the ground and one or more ground rods. A challenge with EM telemetry is that the generated signals are significantly attenuated as they propagate to the surface. Further, the electrical power available to generate EM signals May be provided by batteries or another power source that has limited capacity. Therefore, it is desirable to provide a system in which EM signals are generated efficiently.
Design of the gap sub is an important factor in an EM telemetry system. The gap sub must provide electrical isolation between two parts of the drill string as well as withstand the extreme mechanical loading induced during drilling and the high differential pressures that occur between the center and exterior of the drill pipe. Drill string components are typically made from high strength, ductile metal alloys in order to handle the loading without failure. Most electrically-insulating materials suitable for electrically isolating different parts of a gap sub are weaker than metals (e.g. rubber, plastic, epoxy) or quite brittle (ceramics). This makes it difficult to design a gap sub that is both configured to provide efficient transmission of EM telemetry signals and has the mechanical properties required of a link in the drill string.
There remains a need for ways to support electronics systems at downhole locations in a way that provides at least some protection against mechanical shocks and vibrations and other downhole conditions.